A time division multiplexing (TDM) system is a communication scheme that is currently used. The TDM system has a capacity of up to 40 Gbps, but cannot effectively receive data that are explosively increasing. Thus, new communication networks have been demanded. In recent years, a wavelength division multiplexing (WDM) scheme has been proposed which can easily support several hundreds of Gbps through a single optical fiber.
The WDM system requires a transceiver module that performs an optic-electric conversion and an electric-optic conversion at a subscriber terminal. In particular, since the WDM system is a two-way communication system having a wavelength of 1310 nm and 1550 nm, a filtering element of the transceiver module is a very important element that separates the two wavelengths. Such a filtering element includes a directional coupler, a thin film filter, and a beam splitter. These elements are an effective power routing element. However, the beam splitter (e.g., disclosed in Korean Patent Publication No. 2001-0041551) relates to a two-way optical circulator and a wavelength divider module that can amplify an optical signal in both two ways in order to achieve a two-way optical communication through a single optical fiber. However, the beam splitter has a disadvantage in that it is difficult to integrate the beam splitter and to configure the beam splitter together with a peripheral transmitting/receiving circuit. The thin film filter (e.g., disclosed in Korean Patent Publication No. 2001-0010582) is a wavelength filtering element that deposits a dielectric film having a suitable refractive index on a substrate to reflect or transmit a desired wavelength. The directional coupler (e.g., disclosed in Korean Patent Publication No. 1999-0020073) is a waveguide element that makes two waveguides close to each other and changes a propagating path of an optical signal from one waveguide to the other waveguide using a mode-coupling.
However, even though it is possible to integrate, these elements have a problem in that there is a limitation as to a miniaturization and a manufacturing tolerance is low. Also, a length of a conventional variable optical filter (e.g., disclosed in Koran Patent No. 0438014 and U.S. Pat. No. 6,690,856 B2) is determined by a beat length ratio of an optical wavelength. Therefore, there is a limitation in implementing a miniaturization and a low loss.